list_discussions and get_discussion should include upvoteCount

## `list_discussions` and `get_discussion` should include `upvoteCount`

### Summary

The `list_discussions` and `get_discussion` tools currently return metadata like `title`, `created_at`, `updated_at`, and `comments.totalCount`, but omit `upvoteCount`. This field is available on the `Discussion` type in GitHub's GraphQL API and is the primary signal for gauging community demand on a feature request.

### Use case

When using these tools to triage feature request discussions (e.g. scanning a repo's FR backlog as a PM), vote count is a critical prioritisation input. Without it, the only available proxy for demand is comment count or last-activity date — both of which are noisy signals (a thread with 2 comments and 400 upvotes looks identical to one with 2 comments and 3 upvotes).

### Proposed change

Add `upvoteCount` to the GraphQL query used by both `list_discussions` and `get_discussion`. The field is already on the `Discussion` type:

```graphql
discussion(number: 48) {
upvoteCount # add this
title
createdAt
updatedAt
comments { totalCount }
}
```

No new permissions are required — `upvoteCount` is accessible with the existing `read:discussion` scope.

### Expected output

```json
{
"number": 48,
"title": "Open letter for improving Home Assistant's Authentication system",
"upvoteCount": 412,
"created_at": "2025-06-22T10:18:35Z",
"updated_at": "2026-05-01T09:12:00Z"
}
```